daemon@decwrl.UUCP (02/07/84)
From: erlang::drl (Pack my box with five dozen liquor jugs.) CC: > > Re: 55 saves lives. That's a crock of horse manure! Some people > are so incompetent they can kill at any speed. More people are > killed EVERY year on American highways than during ALL the years > of the Vietnam War (approx 50K me think?) Both of the above two statements ("Some people...", and "More people...") are true, but NEITHER as anything to do with the issue of "55 saves lives". It has been known for a very long time that CHANGE in our environment causes us to be more alert, be more productive, and make less errors. So, lets CHANGE the national speed, Up or down does not really matter much, although down would probably save even more lives. But, the "american public" would never put up with it. > Re: 55 saves gas. That's an even bigger crock! Engines are most > efficient when working at or near their torque peak, be it 30 > mph road speed or 90. My Bunny regularly gets better mileage > at 80 than at 60 (28-29 mpg), and I won't even mention fifty-whot? > Au "Banzai" Nguyen What a field day. Been a long time since I have seen someone stick their neck out this far. RE: "Engines..." This is true. Engines are much, much better when running at or near the torque curve peak. Unfortunatly, the torque curve has nothing to do with this. The optimum thermal efficiency (BTU of fuel burned/BTU of work done by the engine) of any engine is determined by the breathing characteristics, Cam timing, Spark timing, oil temp, water temp, oil viscosity, air temp, and many more things. The only reason the original statment is even close to true is that engines built for everyday use have fairly low, wide torque curves, and you cant help but be close to the "peak" at any time. (A good book on this subject is "Design and Tuning of Competition Engines" put out buy the Bently Press. Bently does lots of car books). As for getting better mileage at 80 vs. 60, BUNK. Drag goes up at the CUBE of velocity, so at 80 one would have to expend about 2.37 times as much fuel to overcome drag as one would at 60. Internal friction in the engine is non-linear also, being somewhere near the square of the engine speed. This implies about 1.78 time as much fuel. Assuming a thermal efficiency of about 30%, and about 20% lost as wasted work, (driving valves and pumps, windage, Etc.), one would need to extract about 1.6 times the number of BTUs from the fuel burned at 80 to provide better mileage that at 60. This implies that the engine be over 60% more efficient with the 33% rpm jump. Not very likely. Also owning and racing VW's for many years (Formula Vee to Formula SuperVee) has led me to believe that the best speed for street tuned water cooled VW's geared as they are in the Rabbit is very close to 50-55, in terms of thermal efficiency (the real thing to measure). Race on the Race tracks (where 80 is fast enough for LAST place), you get to go a lot faster, wheel to wheel with others, you don't have to worry about "some other asshole", they have a doctor just waiting for you to crash (hope you never do!), and its a LOT more fun. Write to: SCCA (Sports Car Club of America) 6750 S. Emporia PO box 3278 Englewood, CO. 80155 Keep the street for driving like a mature adult, even if you don't agree with the speed limit (and I sure don't).
tll@druxu.UUCP (LaidigTL) (02/07/84)
The article describing increased fuel use at higher speeds is generally good, but it does overstate the increase. As for getting better mileage at 80 vs. 60, BUNK. Drag goes up at the CUBE of velocity, so at 80 one would have to expend about 2.37 times as much fuel to overcome drag as one would at 60. Internal friction in the engine is non-linear also, being somewhere near the square of the engine speed. This implies about 1.78 time as much fuel. Although it is true that drag goes up at the cube of velocity, this means that energy needed PER UNIT TIME goes up as the cube of velocity. Since distance covered per unit time goes up linearly with velocity (How's that for an obvious statement?), energy needed per unit distance covered only goes up as the square of velocity. Similarly, energy per unit distance to overcome internal friction only goes up linearly with velocity. Tom Laidig AT&T Information Systems Laboratories, Denver ...!ihnp4!druxu!tll